1. Field of the Invention
The present invention relates to a technique for reducing the noise occurring in an A/D converting process, in an optical time domain reflectometer (OTDR) for measuring the transmission characteristics of an optical fiber from its one end side.
2. Description of the Related Art
Conventionally, an optical time domain reflectometer (OTDR) 10 shown in FIG. 5 has been used for measuring the transmission characteristics of an optical fiber from its one end side.
This OTDR 10 launches an optical pulse emitted from a pulse light source 11 to one end side of an optical fiber 1 as a target object to be tested, through an optical coupler 12 and a port 10a. The OTDR receives the light (backscattered light or Fresnel reflection, etc.) returning to the one end side of the optical fiber 1 in response to the optical pulse, and transmits the light through the optical coupler 12 to an optical receiver 13. An amplifier 14 amplifies an output signal of the optical receiver 13, and inputs the amplified signal to an A/D converter 15. The A/D converter converts the signal to a digital signal having a predetermined number N of bits.
A memory 16 stores a series of digital signals obtained in a predetermined time period beginning from the launch of the optical pulse to the optical fiber 1 until the elapse of a predetermined period of time. A display unit 17 displays waveform data on its display screen, based on the stored N bit digital signals, as shown in FIG. 6. The displayed waveform data gives the attenuation characteristics at each position along the optical fiber 1 or the connection loss.
A controller 18 controls the emission timing of the optical pulse from the pulse light source 11, controls the gain of the amplifier 14, controls the sampling time of the A/D converter 15, and controls the address specification of the memory 16. Further, the controller 18 implements a calculation process (logarithmic process, etc.) for N bit data so as to generate waveform data to be displayed by the display unit 17.
For example, Japanese Patent No. 3002343 discloses the OTDR having the above configuration.
The A/D converter 15 of the thus formed OTDR 10 outputs a voltage in the range of 0 to 1023 mV, for example, where N=10 and a 1 bit equivalent voltage Vr is 1 mV, thus having a dynamic range of 1000 times. However, such the dynamic range of the A/D converter 15 is insufficient by itself for a high optical feedback from a near end of the optical fiber 1 and a low optical feedback from a far end thereof. Therefore, the amplifier 14 amplifies the output signal of the optical receiver 13 using its variable gain, and inputs the signal to the A/D converter 15.
That is, the dynamic range is increased, by decreasing the gain of the amplifier 14 with respect to a high optical feedback from the near end of the optical fiber 1 and by increasing the gain of the amplifier 14 with respect to a low optical feedback from the far end thereof.
The level of Fresnel reflection is greater than that of the backscattered light at least by 40 dB. Because the level of the Fresnel reflection rises so suddenly, it may be difficult to adjust the gain of the amplifier 14 so as to suitably respond to the level of this Fresnel reflection. At this time, an excessive signal which is greater than the maximum input value (2N−1)Vr in the design is input to the A/D converter 15.
When this excessive signal is input, some A/D converter may output a wrong digital signal. Particularly, pipeline A/D converters may output large noise n in the downward direction, as shown by a broken line of FIG. 6. This noise remarkably interferes the measurement. Flash-type A/D converters do not output such noise. However, flash-type A/D converters use a larger amount of power than that used by the pipeline A/D converters, and are expensive, thus are not commonly available or adopted.
One method for preventing the occurrence of the noise in the pipeline A/D converter is to decrease the entire level of the signal to be input to the A/D converter. However, this method involves deterioration of the S/N ratio, and results in a difficulty in analysis of particularly the characteristics of the far end.
Another possible method is to use some A/D converter whose maximum input voltage is larger. In this case however, the number N of bits of output data inevitably needs to be increased in order to obtain required voltage resolution. In addition, it is necessary to newly design following circuits (memory 16 and controller 18) which handle the output data. As a result, this method causes a new problem that the current resources (such as the hardware, software, etc.) cannot be reused.